1. Field of the Invention
This invention pertains to pavement material, pavement construction, and both devices and methods for forming such pavement material and pavement constructions. More particularly, this invention concerns the recycling of asphalt and concrete, and the construction of environmentally sound and safe roads.
2. Description of the Related Art
FIG. 1 is a cross-section of a region of a conventional asphalt pavement. Large aggregate 1 is mixed with small aggregate (or sand) 2, and these components are bound by tar 3. The top of the asphalt pavement 4 is exposed to traffic, while the bottom of the asphalt pavement rests on existing sub-base soil 5.
The top of the asphalt pavement 4 is exposed to the elements, and so is subjected to ultraviolet light from the sun, rain, snow, freezing during winter and thaw during spring. The pavement also experiences frictional wear due to vehicular traffic thereon.
Over time, these environmental and service factors cause the asphalt pavement to deteriorate. As a result, particles of asphalt 4, large aggregate 1, and small aggregate or sand 2 coated with tar 3 may be swept into drainage systems such as pipes and storm inlets. These drainage systems typically discharge into lakes, rivers, streams or oceans. As will be explained hereafter, such pavement particles contribute to water pollution.
The specific gravity of the tar particles 3 is very close to that of water, which has a specific gravity of one. Accordingly, when rainwater, sleet, and melting snow drain from the asphalt pavement, the tar particles 3 float and are swept into the bodies of water, such as lakes, rivers, streams and oceans, which receive the water discharge.
Tar particles 3 that are attached to small aggregates such as sand 2 are usually carried off into drainage systems such as pipes, culverts and storm inlets, where they settle to the bottom, since the tar and aggregate together have a specific gravity greater than that of the water.
Over time, as the asphalt pavement continues to deteriorate, a considerable quantity of aggregates 1 and 2 coated with tar 3 accumulates at the bottom of bodies of water, thereby depleting the oxygen in the water, and reducing the amount of aquatic life. In addition, the tar may contain heavy metals, which have already been shown to have other adverse effects. The results of such pollution are well-known, and it can markedly affect the health of wildlife and humans.
The continued degradation of asphalt pavement often results in pot-holes, which are major structural failures of the asphalt pavement. Asphalt material breaking away from these potholes also causes accelerated pollution of water resources in the manner already discussed.
A substantial amount of the water pollution which enters ground water, whether in aquifers, streams, rivers, bays or oceans, therefore originates as runoff from asphalt and concrete roads and parking lots. Wherever there are asphalt or concrete roads, driveways, or parking lots, pavement-based pollutants are likely to be introduced into the water runoff.
These water pollutants include hydrocarbons, which are inherently present in asphalt pavement. Asphalt pavement is composed of tar, sand and rock aggregates. The tar component of an asphalt road functions as a binder which holds the aggregate particles together. Tar is a thermo-plastic material, and it deteriorates when exposed to heat and/or ultraviolet energy. Since any asphalt used outside is continually exposed to heat and ultraviolet energy from the sun, that asphalt eventually degrades. Furthermore, vehicle traffic also wears down the asphalt road surface. As a result of both the thermal/ultraviolet degradation and physical wear, components of the asphalt road are released. As previously explained, these components are carried by storm water runoff into roadside waters and ditches which lead through stormwater systems to large bodies of water such as lakes, rivers, bays or oceans, and from there, into drinking water aquifers.
Still other sources of pavement degradation are the freeze-thaw cycles experienced during winter seasons, and the pumping action produced by vehicle traffic. Both of these phenomena physically flex the roadways and so cause pavement degradation, which further undermines both asphalt and concrete pavement.
Both asphalt and concrete roads also contain oil drippings which have fallen from passing vehicles. These oil drippings, which are eventually flushed into water outfalls, are a further source of water pollution.
Thus, tars, pavement particles, and vehicular oil drippings, whether on asphalt or concrete pavement, all of which are polluting substances, are carried into stormwater systems such as sewers, ditches, culverts, etc., from where they are ultimately deposited into streams, lakes, rivers, bays and oceans, so that they can enter the drinking water or food supplies. Using retention ponds to store stormwater is of no benefit; such ponds grow polluted and they become another source of ground water polluted by hydrocarbons and heavy metals.
It has been determined that hydrocarbon pollutants entering the water supply create serious health problems for both humans and wildlife. Such hydrocarbons are particularly detrimental when they enter the drinking water supply, since many water treatment plants add chorine to the water to kill bacteria. Chlorination of the drinking water creates conditions which lead to the production of chlorinated hydrocarbons. Chlorinated hydrocarbons are carcinogenic, so the presence of such cancer-causing chemicals is clearly undesirable.
Water treatment plants are not usually equipped to remove hydrocarbons, sulfur or dissolved metals from drinking water. Accordingly, by eliminating or at least reducing the amount of hydrocarbons, sulfur and heavy metals emanating from asphalt roads, adverse effects on drinking water can be reduced.
Still another reason why asphalt roads contribute to water pollution is because road tar contains heavy and light metals and sulfur. Heavy metals (i.e., copper, lead and mercury) and light metals (i.e., aluminum and arsenic) have been shown to be detrimental to the health of man and wildlife, especially if they enter the food chain. Sulfur, when combined with rainwater, acidifies the water, and the acidic runoff in turn acidifies lakes, rivers, streams, bays and even oceans. While the presence of such acids in water has been attributed to acid carrying clouds, which produce "acid rain," asphalt roads also contain sulfur which can be flushed out and contribute to acidic runoff. This acidic runoff, flushed from parking lots, streets and highways, is another source of acid water, which is perceived as the phenomenon "acid rain".
It is therefore essential that the tar in asphalt roads be rendered environmentally safe as a binding element. One way to do this would be to recycle the road material into an environmental non-polluting porous pavement.
Even non-polluting water runoff can be troublesome, because of the flooding which can result. This is a particular concern in newly-developed areas, where the construction of residential and business sub-divisions, shopping centers and roads may lead to increased water runoff. If the runoff exceeds the area's drainage capacity, flooding can occur.
A further concern in the design of roadways is the need to avoid having standing water on the roadway. It has been determined that the majority of skidding accidents occur because water has accumulated on the roadway. When a vehicle passes over such standing water, the water may accumulate under one or more of the vehicle's tires. This causes hydroplaning, which is a situation where the vehicle tire loses contact with the road surface, and "floats" on a film of water. Needless to say, hydroplaning is quite dangerous, since the ensuing skidding can cause a loss of control.
Thus, there is a substantial need to provide roads which do not allow standing water to form thereon.
Still another concern in designing roads arises because conventional asphalt and concrete roads are composed of stone aggregates and sand which become polished by vehicle traffic. This polishing reduces the coefficient of friction between vehicles tires and the road, making skidding more likely.
Moreover, asphalt roads contain oils, which further reduces the coefficient of friction between tires and the asphalt roadbed. In addition, oil drippings which accumulate on the surface of either asphalt or concrete roads also can cause skidding.
Accordingly, it is highly desirable to provide roads in which there is a high coefficient of friction between the road surface and the tires of passing vehicles.
Another aspect of this invention relates to improving the types of trucks which are used to process road paving material as it is shipped to the location where it will be formed into pavement. Conventional bulk mixing machines, which are known as transit mix trucks, are designed to mix conventional concrete. These machines are designed to mix relatively high slump materials, high meaning slumps ranging from 1-4. Lower slumps of concrete, i.e., concrete containing lower water-cement ratios, are difficult to produce using transit mix trucks, since such trucks contain angularly positioned rotatable drums with fixed internal blades that mix the concrete as it flows over the blades of the rotating drums. If one tries to produce low slump low moisture concrete in these machines, one ends up producing inhomogenous balls of concrete which have undergone little or no mixing. Furthermore, transit mix trucks are not designed to handle polymeric admixtures, as described throughout this application. Since such admixtures would render the mixing machines "sticky", the addition of such admixtures would cause cement to adhere to the inside surface of the truck's drum, rendering it virtually impossible to clean, and possibly even damaging the transit-mix truck.
Another conventional concrete mixing machine is known as a mobile mixer. Mobile mixers carry dry sand, stone aggregates, water and admixtures in separate compartments which are arranged on a truck bed. When the mobile mixer arrives at a construction site, the materials are batched mechanically, usually through a series of gates and valves. Mixing is performed by a mixing augur provided at the rear of the machine. One disadvantage to using these mobile mixers is that the augur must rotate continuously, and can never stop during processing, because the augur must rotate in order to achieve a homogenous mix. This, however, is nearly impossible to achieve, since whenever the machine is moved to different job sites, the augur must be stopped so that concrete will not pour out from the truck during relocation. Accordingly, the mobile mixer, while superior to the transit mix truck for batching low slump mixes, nevertheless suffers from the same disadvantages that were discussed previously with regard to the mobile mixers, namely, when the machine is stopped for transport, dry and non-homogenous concrete results.
While there are other types of mixing machines commercially available on the market, such machines are of quite limited mobility, or they otherwise suffer from at least some of the shortcomings of the transit mix truck and/or the mobile mixer.
Still another disadvantage of both the transit mix truck and the mobile mixer is that, once they have run out of concrete, they usually must be returned to a batch plant to be replenished with a fresh supply of material. These machines are therefore not well-suited for on-site batching and mixing.
Furthermore, the batching and mixing of recycled asphaltic products ("RAP") are not easily accomplished using currently available equipment.